1. Field of the Invention
The present invention relates to a scanning electron microscope, particularly to a scanning electron microscope in which a reduction in detection efficiency of electron emitted from a specimen is prevented.
2. Background Art
A scanning electron microscope is a device for forming an image of a scanned specimen, by focusing a primary electrons beam emitted from an electrons source using a condenser lens and an objective lens, scanning the electrons beam over a specimen and irradiating the specimen to cause secondary electron emission from the specimen, detecting the emitted secondary electrons, and synchronizing a detection signal of the secondary electrons with a scanning of the electrons beam for an imaging process.
In such a scanning electron microscope, a detection efficiency of secondary electrons is an important factor for increasing an S/N (signal-to-noise ratio) of a specimen image. An increased detection efficiency of secondary electrons requires an increased amount of electrons emitted from the specimen and detected by the scanning electron microscope.
The electrons emitted from a specimen have various types of energies. Generally, the electrons having energy up to 50 eV is called secondary electron, while the electrons having energy above 50 eV is called backscattered electron. A detector is applied a positive pull-in voltage on the order of 10 kV for pulling in electrons (especially, low-energy secondary electrons) emitted from a specimen.
Meanwhile, a scanning electron microscope is used for example by accelerating primary electrons through a magnetic field of an objective lens, as an approach for enhancing a resolving power for imaging a specimen. In order to accelerate the primary electrons, an electrode is provided to an objective lens, or a pole piece of an objective lens is electrically insulated and a positive voltage is applied thereto. In the latter case, a misalignment between a magnetic field produced by the objective lens and an electric field generated by the application of the voltage can be decreased, resulting in enhancement of a resolving power (see JP Patent Publication (Kokai) No. 9-171791A (1997), for example).
However, in this case, the electrically insulated gap is also magnetically insulated, which leads to a leakage of the magnetic field across the gap. If such a leaked magnetic field exists between a specimen and a detector, the leaked magnetic field causes an electron trajectory to the detector of electrons emitted from the specimen toward the detector to be curved, and a less amount of the electrons reach the detector. This results in a problem of a reduced S/N ratio of the specimen image.